In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by bringing an electrographic developer into contact with the latent image. If desired, the latent image can be transferred to another surface before development.
In latent-image formation the imagewise discharge is brought about by the radiation-induced creation of pairs of negative-charge electrons and positive-charge holes, which are generated by a material (often referred to as a charge generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials included in the electrophotographic element, typically, either the holes or the electrons that have been generated migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating electron/hole pairs in response to exposure to actinic radiation and is referred to as a charge generation layer (hereinafter sometimes also referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge generation layer and is referred to as a charge transport layer (hereinafter sometimes also referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. The CGL comprises at least a charge generation material; the CTL comprises at least a charge transport material (a material which readily accepts holes and/or electrons generated by the charge generation material in the CGL and facilitates their migration through the CTL in order to cause imagewise electrical discharge of the element and thereby create the electrostatic latent image); and either or both layers may additionally comprise a film-forming polymeric binder.
Many multiactive electrophotographic elements currently in use are designed to be charged initially with a negative polarity and to be developed with a positively charged toner material. Usually, the arrangement of layers in such elements has the CGL situated between the CTL and the electrically conductive layer, so that the CTL is the uppermost of the three layers, and its outer surface bears the initial negative charge (although in some cases there may be a protective overcoat over the CTL which bears the initial charge). Such elements contain a charge transport material in the CTL which facilitates the migration of positive holes (generated in the CGL) toward the negatively charged CTL surface in imagewise exposed areas in order to cause imagewise discharge. Such material is often referred to as a hole transport material. In elements of that type a positively charged toner material is then used to develop the remaining imagewise unexposed portions of the negative-polarity potential (i.e., the latent image) into a toner image. Because of the wide use of negatively charging elements, considerable numbers and types of positively charging toners have been fashioned and are available for use in electrographic developers.
However, for some applications of electrophotography it is more desirable to be able to develop the surface areas of the element that have been imagewise exposed to actinic radiation, rather than those that remain imagewise unexposed. For example, in electrophotographic printing of alphanumeric characters it is more desirable to be able to expose the relatively small percentage of surface area that will actually be developed to form visible alphanumeric toner images, rather than waste energy exposing the relatively large percentage of surface area that will constitute undeveloped background portions of the final image. In order to accomplish this while still employing widely available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Thus, positive toner can then be used to develop the exposed surface areas (which will have relatively negative electrostatic potential after exposure and discharge, compared with the unexposed areas, where the initial positive potential will remain).
A multiactive electrophotographic element can be designed to be charged positively initially and still have the layer arrangement wherein the CGL is situated between the CTL and the electrically conductive layer. However, such an element must contain an adequate electron transport agent (i.e., a material which adequately facilitates the migration of photo-generated electrons toward the positively charged insulative element surface) in its CTL. While many materials having good hole-transport properties have been fashioned for use in electrophotographic elements, unfortunately, relatively few materials are known to provide good electron transport properties in electrophotographic elements.
A number of chemical compounds having electron transport properties are described, for example, in U.S. Pat. Nos. 4,175,960; 4,514,481; 4,474,865; 4,546,059; 4,227,551; 4,609,602; 4,869,984; 4,869,985; 4,913,996; 4,997,737; 5,034,293; and 5,039,585.
Some prior art electron transport agents do not perform the electron transporting function very well, especially under certain conditions or when included in certain types of electrophotographic elements.
Some of such elements containing prior art electron transport agents exhibit poor charge acceptance. The phrase, "charge acceptance," refers to the capability of the element to be charged initially to the desired level of uniform potential at the beginning of each cycle of its normal operation (a cycle being the sequence of operation comprising initially uniformly charging the element, exposing the element imagewise to actinic radiation to form the electrostatic latent image, optionally developing the electrostatic latent image into a toner image with an electrographic developer, and erasing the remaining potential on the element to prepare it for the next cycle of operation). "Poor charge acceptance" means that the element has a relatively poor capability of being initially charged to the desired level of potential.
Also some prior art electron transport agents cause an undesirably high rate of discharge of the electrophotographic element before it is exposed to actinic radiation (often referred to as high dark decay).
Some multiactive elements containing known electron transport agents exhibit photosensitivity that is lower than desirable. The term, "photosensitivity" (sometimes referred to as "electrophotographic speed") refers to the amount of incident actinic radiant energy to which the element must be exposed in order to achieve the desired degree of discharge of the initial potential to which the element was initially charged. The lesser the amount of radiant energy required for such discharge is, the higher is the photosensitivity, and vice versa.
Some known electron transport agents provide relatively poor (i.e., low) electron mobility in CTL's. The term, "electron mobility," refers to the velocity with which the electron transport agent will transport electrons (that were generated in the CGL) through the CTL to cause imagewise discharge of the initial uniform potential on the element. Higher electron mobility enables the photogenerated electrons to traverse the CTL and cause the discharge in a shorter period of time. High electron mobility enables use of an element, for example, in a high speed copier employing high-intensity, short-duration imagewise exposure (commonly also referred to as flash exposure), wherein the time it will take for the element to properly discharge, and, thus, the length of the period needed between the end of the exposure step and the beginning of the toner image development step, is determined by the level of electron mobility within the element. The higher the mobility is, the shorter is the necessary waiting period between exposure and development, and the greater is the number of copies that can be made in a given amount of time.
Also, some known electron transport agents comprise compounds known to be toxic or carcinogenic (e.g., 2,4,7-trinitrofluorenone).
In general, there are simply not enough known relatively good electron transport agents available to choose from in order to have the flexibility to be able to develop electrophotographic elements that photodischarge by means of electron transport and that can be optimized for use in various different situations (e.g., where an element is desired to contain certain charge generating materials, sensitizers, binders, conducting layers, etc., or where it is desired to charge the element with a certain polarity or level of charge, to subject the element to imagewise exposure at a particular wavelength or intensity of radiation, to use the element in copiers that require it to photodischarge in a certain time or require it to be able to hold a charge in darkness for a particular period of time before imagewise exposure, etc.).
Thus, there is a continuing need for new electron transport agents for multiactive electrophotographic elements, in order to have the flexibility to meet the above-noted needs, namely, to be able to fashion multiactive elements that can discharge by means of electron transport and can exhibit good combinations of performance properties such as good charge acceptance, dark decay, photosensitivity, and electron mobility.